1. Technical Field
The present invention relates to a white ink composition for inkjet recording, an ink set for multilayer formation including the white ink composition, an image forming method using the white ink composition, and a printed matter formed by the white ink composition.
2. Description of the Related Art
As image recording methods for forming an image on a recording medium such as paper based on an image data signal, there are electrophotographic systems, sublimation type and melt type thermal transfer systems, inkjet systems, and the like.
Inkjet systems use inexpensive devices to eject ink on only a necessary image area so as to directly form an image on a recording medium. Accordingly, the inkjet systems allow efficient use of ink and have an advantage of low running cost. In addition, inkjet printing is less noisy and excellent as an image recording system.
Among ink compositions used for image recording by inkjet systems, an active energy ray curable aqueous ink composition is a technology having many excellent characteristics and potentials. For example, the composition can be suitably used for image printing, pre-treatment for providing printability to a recording medium, post-treatment for protection and decoration of a printed image, and the like. In addition, the composition mainly composed of water is highly safe, and the decreased viscosity of the composition allows for the application thereof to high density inkjet recording.
In recent years, ink images formed by ink composition on recording media made of resin have increasingly been used for purposes such as large-area advertisements and outdoor advertisements.
The greatest advantage of inkjet systems is their suitability for forming wide printed matters. As image drawing systems for inkjet printing, there are a single-pass system and a shuttle-scan system. The former is a system in which an inkjet head having a paper width is prepared and a recording medium is moved under the head to print an image on the medium. The single-pass system has an advantage of enabling high speed printing. However, for wide drawing, the system is disadvantageous, since it is necessary to arrange the width of the inkjet head over the width of the drawing, device cost increases. On the other hand, the shuttle-scan system is a system in which an inkjet head with a narrow width is moved in a direction intersecting with respect to a paper moving direction to perform drawing. This is not suitable for purposes requiring high speed drawing, as in the single-pass system, but is an effective drawing system in terms of reducing device cost in drawing on a wide base material. Such a shuttle-scan drawing method is mainly used in the fields of signs and displays.
Particularly, for outdoor advertisements, images needs to be highly durable. Additionally, in order to successively form large-area printed matters or many sheets of printed matter, it is necessary to perform a step of removing stains on the printer by use of an organic solvent. However, when the organic solvent used for cleaning has scattered and adhered onto a formed ink image, if it is an image having low solvent resistance, the portion of the image on which the solvent has been adhered is dissolved and removed, causing image defects such as missing nozzles. Accordingly, there has been a desire for an ink composition capable of forming an image having high levels of both water resistance and solvent resistance.
In order to use for signs and displays, it has been necessary to enable drawing on various base materials. Examples of plastic base materials include polyvinyl chloride (PVC), polyethylene terephthalate (PET), various polyesters, and polyolefins such as polyethylene and polypropylene. One of effective printing methods for signs and displays is to use a transparent base material for printed matter. However, it has been difficult for printed matter obtained by drawing on a transparent base material to have sufficient image density because of the optical transparency of the base material itself.
Accordingly, in order to further improve the sharpness of an image formed on a transparent base material or a light-colored base material, it is considered preferable to apply a white ink composition in advance on a region of a recording medium where a color ink composition is to be applied, thereby providing the so-called white backing Such a white ink composition is required to have high concealability.
As a white pigment having high concealability, titanium dioxide is known.
Titanium dioxide, however, tends to precipitate, and once precipitated, it is difficult to re-disperse the substance. Thus, there has been proposed a white ink composition that facilitates re-dispersing of titanium dioxide even when it precipitates and hardly causes clogging of nozzles of a recording head in an inkjet recording apparatus (for example, see Japanese Patent Application Laid-Open (JP-A) No. 2008-308692 and JP-A No. 2012-97214).
On the other hand, in an ink composition for inkjet recording including a magenta pigment or a cyan pigment, in order to reduce coagulation of the pigment, there has been proposed a technique of adding a natural or synthetic smectite clay mineral (JP-A No. 2004-190032).